The present invention relates to a process for producing a magnetite-coated electrode which exhibits an excellent durability when used in electrolysis, etc., under severe conditions.
Various oxide materials have hitherto drawn attention as a material suitable for an insoluble electrode for use in various types of electrolysis, etc. Among them, magnetite has been believed to be the most practical material.
Magnetite is a ferromagnetic substance having an inverse spinel structure and a composition of Fe.sub.3 O.sub.4 and exhibits excellent electrical conductivity and corrosion resistance.
Accordingly, a magnetite powder has been molded into a columnar, plate or cylindrical article by casting or sintering for use as an electrode.
Later on, an electrode made from a metallic material having excellent workability and electrical conductivity and coated with magnetite has been developed for the purpose of further improving the workability and electrical conductivity of the magnetite electrode.
For example, Japanese patent application Kokai publication No. 53-103980 discloses a process for producing a magnetite-coated electrode which comprises subjecting an electrically conductive, corrosion-resistant metal substrate, such as titanium, zirconium, tantalum or niobium, to plasma spray coating by making use of a magnetite powder having a composition comprising Fe.sub.2 O.sub.3 and FeO in an Fe.sub.2 O.sub.3 to FeO weight ratio of 2.4 to 2.8 in a gas stream of a reducing atmosphere to coat the substrate with magnetite, immersing the coated substrate in an aqueous iron salt solution under a reduced pressure, and heating the immersed coated substrate in a mixed gas mainly composed of steam at a temperature of 550.degree. to 700.degree. C.
This method, however, involves a complicated process and further has the following drawbacks due to the use of cast magnetite, natural magnetite or synthetic magnetite as the spray coating source.
Specifically, the cast magnetite and natural magnetite contain large amounts of impurities such as silica, which causes the resultant magnetite coating to become porous, the strength to be lowered and the electrical specific resistance to be enhanced.
The synthetic magnetite is usually in the form of an ultrafine particle having a size of 1 .mu.m or less. Therefore, it clogs the pipe of a powder feeder, so that the spray coating becomes impossible. In this case, even if the spray coating could be conducted with difficulty, the adhesion of the powder lowers or the resultant coating is so dense that the internal stress increases and peeling may occur, which makes it difficult to form a thick coating.
In the present invention, the above-described problems are solved by utilizing a pulverized magnetite sinter prepared under particular conditions as the spray coating source.